The present invention relates generally to leakage detectors, and more particularly, is directed to a leakage detector which can be used in combination with a signal level meter.
The use of a signal level meter (SLM) to measure signal levels of a cable or CATV system, as well as leakage from such a system, is well known. For example, in the article xe2x80x9cFCC Leakage Limitsxe2x80x9d in Communications Technology, July, 1988, page 38 by Martin J. Walker, it is stated that the FCC limits on leakage are 15 xcexcV/m at 100 feet for frequencies up to and including 54 MHz or frequencies over 216 MHz, and 20 xcexcV/m at 10 feet over 54 MHz up to and including 216 MHz. In this article, Mr. Walker states that most systems use a signal level meter calibrated in dBmV to measure leakage.
NCTA Recommended Practices for Measurements on Cable Television Systems, Second Edition, 1989, published by the National Cable Television Association (NCTA), Washington, D.C., pages I.J.1-I.J.10, deals with signal leakage systems which can be used to comply with FCC Rules and Regulations part 76, subpart K, as to the maximum leakage limits from a cable system. As stated at page I.J.6, most measurements of field strength are commonly made with a field strength meter which is, in fact, a selective voltmeter. However, as further discussed therein, most commercially available meters do not have the required sensitivity to measure 20 xcexcV/m at channel 13, and therefore, measurements may have to be confined to either the low end of the VHF high band or by employing a preamplifier ahead of the field strength meter. At page I.J.8, there is also a teaching of decreasing the IF bandwidth of a spectrum analyzer so that less noise energy from external or man-made noise is admitted, whereby such external or man-made noise, for example, from an urban environment, does not obstruct or affect the measurement of leakage.
U.S. Pat. No. 5,493,210 to Orndorff et al, and U.S. Pat. No. 5,633,582 to Orndorff et al, which is a continuation of the former, the disclosures of which are both incorporated herein by reference, use the above teachings to provide a combined signal level meter and leakage detector. In these patents, a signal level meter is also used to detect leakage, and a common IF stage is provided for both leakage detection and the signal level meter operation. Specifically, the required sensitivity for leakage detection is provided in two ways. First, the IF stage includes two switchable IF filters, one having a bandwidth of 280 KHz for use in the signal level meter mode, and the other having a reduced bandwidth of 10 KHz for the leakage detection mode. Switching is controlled by a microcontroller. Second, a preamplifier is provided between the antenna which receives the leakage signal and the first mixer in the IF stage for the same. Since the mixer adds noise to the signal, the preamplifier amplifies the leakage signal before the noise is added by the mixer, thereby increasing the signal to noise ratio.
However, the system of these patents is relatively complicated, and requires two separate inputs, one for the antenna which is used in the leakage mode, and the other for the cable which is used in the signal level meter mode. As a result, two different receiver front ends are required, one being a wideband dual-conversion receiver front end connected to the cable input and the other being a single-conversion receiver front end connected to the antenna input. The outputs of the two receiver front ends are connected with two different inputs of the IF stage. This arrangement, however, makes the circuitry more complicated and costly.
For a video modulated carrier, the 100% carrier level corresponds to the vertical and horizontal sync peaks. The carrier level within the video portions is variable and depends on the picture information being transmitted. Therefore, the most accurate way to measure the level of the video modulated carrier for signal level meter and leakage detector applications is to determine the peak level at the horizontal or vertical sync peaks.
Further, since the leakage signal can be 10-20 dB below the level of the noise floor, the required measurement range for leakage measurements must extend to signals to about xe2x88x9255 dBmV or lower. As discussed above, the classic way around this problem is to use a narrower IF passband and to also increase the level of the leakage signal prior to adding noise by the receiver front end. This means that the signal level meter must use an IF bandwidth of 10-15 KHz to improve sensitivity for the leakage measurement. However, this has the drawback that the lowered bandwidth does not allow the peaks of the horizontal synchronizing signal to be used for processing, so that the speed of response to signal level changes is fundamentally limited to the detection and processing of peaks of the vertical synchronizing signal or to an average of the video signal. To use the peaks of the horizontal synchronizing signal, a wide IF bandwidth of, for example, 280 KHz must be used for leakage detection.
Also, the use of a narrower IF bandwidth requires greater tuning precision in the tuner and in the electronic tuning control.
In the former case, detection of the peaks of the vertical synchronizing signal will not give a sufficiently useful response for leak tracing because of the slowness of the system. In the latter case of detecting the average of the video signal, the measurement variation of the leakage signal is unrelated to the carrier level of the video signal, due to constant changes in the video signal. As a result, leakage detection is affected by the video signal itself, which can provide misleading results.
Accordingly, it is an object of the present invention to provide a leakage detector which can be used in combination with a signal level meter, that overcomes the problems with the aforementioned prior art.
It is another object of the present invention to provide a combination leakage detector and signal level meter that includes a single signal path, and thereby, common circuitry, for both signal level meter and leakage detection operations.
It is still another object of the present invention to provide a leakage detector which provides leakage signal processing circuitry after the IF stage and detector for use in leakage detection operations.
It is yet another object of the present invention to provide a leakage detector which uses filtering and noise power subtraction circuitry after the IF stage and detector for use in leakage detection.
It is a further object of the present invention to provide a leakage detector having a sufficiently fast response to signal level changes, in order to use the peaks of the horizontal synchronizing signal of the video signal to reduce noise in the leakage signal, and thereby easily trace leaks in a CATV system.
It is a still further object of the present invention to provide a leakage detector which uses a relatively wide IF bandwidth of 280 KHz for both the signal level meter mode and leakage detection mode.
It is a yet further object of the present invention to provide a leakage detector which can be used in combination with a signal level meter, and which is economical and easy to manufacture and use.
In accordance with an aspect of the present invention, a leakage detector includes a receiver front end having an input for connection with an antenna; an intermediate frequency (IF) stage connected with the receiver front end for producing an intermediate frequency signal; a detector having an input for producing an amplitude modulation (AM) detected output signal in response to the intermediate frequency signal; a leak processor operative in a leakage mode only when the antenna is connected with the receiver front end, the leak processor having an input connected to the detector for removing noise from the AM detected output signal which is above and below the frequency of a synchronizing signal of the AM detected output signal, and which is not in frequency coherence with the synchronizing signal of the AM detected output signal, the synchronizing signal having a peak level, and the signal processor providing an output signal corresponding to the peak level of the synchronizing signal of the AM detected output signal; a display; and a processing unit connected with the leak processor and the display for displaying leakage data in response to the output signal from the leak processor.
The leak processor includes a filter having an input connected with the detector for removing AC noise from the AM detected output signal which is not in frequency coherence with the synchronizing signal, and for producing a filtered AM detected output signal; a first peak detector having an input connected with the detector for receiving the AM detected output signal therefrom and producing a first peak detected output signal; a second peak detector having an input connected with the filter for receiving the filtered AM detected output signal therefrom and producing a second peak detected output signal; and a noise power subtraction circuit having inputs connected with the first and second peak detectors for subtracting a DC level equivalent to noise power from the filtered AM detected output signal in response to the first and second peak detected output signals, to provide the output signal corresponding to leakage.
The noise power subtraction circuit includes a first difference circuit having a first input connected with the first peak detector and a second input connected with the second peak detector, and which produces an output error signal in response to a difference between the first and second peak detected output signals; and a second difference circuit having a first input connected with the first difference circuit and a second input connected with the second peak detector, and which produces the output signal corresponding to leakage in response to a difference between the first and second peak detected output signals.
The leak processor further includes an equalization circuit connected between the filter and the second peak detector for compensating for a reduction in signal level of the filtered AM detected output signal from the filter. The synchronizing signal is a horizontal synchronizing signal, and the filter includes an N-path filter clocked at a multiple rate corresponding to a sweep rate of the horizontal synchronizing signal.
Further, the leak processor includes a gate circuit, along with a gate control circuit having an output connected with an input of the gate circuit, and which controls the gate circuit to pass the output signal corresponding to leakage to the processing unit only when the AM detected output signal includes a video modulated carrier component. The gate control circuit includes a filter which only passes that portion of the AM detected output signal at a frequency of the synchronizing signal; a detector having an input connected with the filter, and which detects the presence of an output signal from the filter to produce a control signal; and a gate drive circuit having an input connected with the detector and an output connected with the gate circuit, for controlling the gate circuit to pass the output signal corresponding to leakage to the processing unit only when the AM detected output signal includes the video modulated carrier component.
A speaker is provided for producing an audio output; and an audio circuit supplies the output signal corresponding to leakage to the speaker.
The leak processor includes a voltage controlled oscillator (VCO) driven by the output signal from the leak processor and which supplies a signal to the audio circuit such that the speaker produces an audio feedback tone having a pitch that increases with increasing level of the output signal corresponding to leakage and decreases with decreasing level of the output signal corresponding to leakage. The pitch of the audio feedback tone will vary with leakage intensity and the leakage intensity will vary with orientation of the antenna relative to, and distance of the antenna from, the leakage. Further, the processing unit disables the VCO when the output signal corresponding to leakage is below a threshold level.
Thus, the leak processor includes a VCO which is driven directly from the output signal corresponding to the leakage level. The VCO produces a signal which is used for audio feedback as an aid to leak tracing. The leak processor output is sufficiently accurate to be used as the direct control of the VCO frequency. The output signal from the VCO is fed to the audio amplifier and speaker in the unit to produce the audio feedback tone. The pitch of the tone increases with increasing leak signal level, and decreases with decreasing leak signal level. Due to the fast response of the leak processor, all variations in leak signal level, including additional AM modulation, can be heard via the tone.
In accordance with another aspect of the present invention, a combination signal level meter and leakage detector, includes a single receiver front end having an input for connection with an antenna or a cable for a CATV signal; an intermediate frequency (IF) stage connected with the receiver front end for producing an intermediate frequency signal; a detector having an input for producing an amplitude modulation (AM) detected output signal in response to the intermediate frequency signal; a leak processor operative in a leakage mode only when the antenna is connected with the single receiver front end, the leak processor having an input connected to the detector for removing noise from the AM detected output signal which is above and below the frequency of a synchronizing signal of the AM detected output signal, and which is not in frequency coherence with the synchronizing signal of the AM detected output signal, the synchronizing signal having a peak level, and the signal processor providing an output signal corresponding to the peak level of the synchronizing signal of the AM detected output signal; at least one signal detector for detecting characteristics of the CATV signal during a signal level meter mode, when the cable is connected with the single receiver front end; a display; and a processing unit connected with the leak processor, the at least one signal detector, and the display for displaying leakage data in response to the output signal from the leak processor during the leakage mode and for displaying the characteristics of the CATV signal during the signal level meter mode.
The at least one signal detector preferably includes a peak detector, an average detector and a hum detector.
Also, the single receiver front end is a wideband dual-conversion receiver front end, which includes a first heterodyne mixer which converts an input signal thereto to a first intermediate frequency signal; and a second heterodyne mixer which converts the first intermediate frequency signal to a second intermediate frequency signal which is supplied to the detector.
The IF stage maintains an IF bandwidth of 280 KHz for both the leakage mode and the signal level meter mode.
The detector further produces a detected frequency modulation (FM) output signal; and further includes a speaker for producing an audio output, and an audio amplifier for supplying the detected frequency modulation output signal to the speaker. A mute circuit is connected between the detector and the audio amplifier for preventing passage of the detected frequency modulation output signal, to the audio amplifier during the leakage mode.
In accordance with still another aspect of the present invention, a video signal detector includes a receiver front end having an input for connection with an antenna or cable; an intermediate frequency (IF) stage connected with the receiver front end for producing an intermediate frequency signal; a detector having an input for producing an amplitude modulation (AM) detected output signal in response to the intermediate frequency signal; a signal processor having an input connected to the detector for removing noise from the AM detected output signal which is above and below the frequency of a synchronizing signal of the AM detected output signal, and which is not in frequency coherence with the synchronizing signal of the AM detected output signal, the synchronizing signal having a peak level, and the signal processor providing an output signal corresponding to the peak level of the synchronizing signal of the AM detected output signal; a display; and a processing unit connected with the signal processor and the display for displaying video signal level data in response to the output signal from the signal processor.
The above and other objects, features and advantages of the invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.